How to inspect spark plugs in car

Think of spark plugs as tiny windows into what’s happening inside your engine’s combustion chambers. Their main job is simple: create a spark at just the right moment to ignite the mix of fuel and air. 

But because they’re right in the middle of all that action, their condition and appearance can give you awesome clues about your engine’s health!  

Inspecting your spark plugs is a classic diagnostic trick. It doesn’t require fancy electronic tools, just a bit of elbow grease and the ability to look closely. 

What they look like after some time in your engine tells a story about how that specific cylinder is burning fuel and oil.  

Here’s how to get started and what to look for:

1. Cool Down: Always make sure your engine is completely cool before attempting to remove spark plugs. Removing them from a hot aluminum cylinder head can damage the threads. 
   
2. Gather Tools: You’ll need a spark plug socket (they have a rubber insert to protect the plug), a socket wrench, and maybe an extension to reach the plugs.
   
3. Remove Safely: Carefully disconnect the spark plug wire or ignition coil connected to the plug. Don’t yank on the wire itself! Use your socket and wrench to gently unscrew the plug from the cylinder head. Keep track of which plug came from which cylinder – working on one cylinder at a time (remove plug, inspect, replace, move to next) helps prevent mixing things up. 
   
4. Inspect the Tip: Once the plug is out, look closely at the firing end – the electrode and the insulator tip around it. This is where the story is told!
   
   • Normal: The tip should look light tan, grayish, or brownish. There might be light deposits, but they won’t be thick or greasy. This is a sign of a healthy engine, proper air/fuel mix, and the correct spark plug “heat range” for your engine.

• Fouled (Dry, Black Soot): If the tip is covered in dry, black, fluffy carbon (soot), the cylinder is likely running “rich” (too much fuel, not enough air), or maybe has a weak spark that isn’t fully burning the fuel.
  
• Fouled (Wet, Black/Brown Oil): A black, wet, oily look means oil is getting into the combustion chamber. This often indicates wear in the cylinder like worn piston rings, valve guides, or valve seals. 

• White/Ashy Deposits: Hard, white, or light gray crusty deposits can indicate additives in fuel or oil are burning. Sometimes linked to oil consumption or fuel quality issues.
  
• Melted or Damaged Electrodes: If the tip looks blistered, melted, or severely eroded beyond normal wear, the plug has been running too hot. This is serious and could be caused by incorrect ignition timing, a plug with too hot a “heat range” for your engine, a lean fuel mixture (not enough fuel), or even cooling system problems. 

• Worn Electrode: Over time, the sharp edges of the electrode wear away and the gap might increase. This is normal wear, but if severe, indicates it’s time for new plugs.

5. Compare Them All: The most valuable part of this test is comparing all the spark plugs from your engine. If most look normal but one or two look significantly different (fouled, oily, or damaged), that immediately tells you the problem is isolated to those specific cylinders.
   

Inspecting your spark plugs is a simple yet powerful diagnostic step that can quickly point you towards the root cause of many engine performance issues, misfires, or smoke problems.  

Give your spark plugs a look next time something doesn’t feel quite right with the engine!

Normal: Indicates a healthy engine. The light tan or gray insulator and minimal deposits show proper combustion and correct heat range.

Dry Fouling: Caused by a rich fuel mixture, weak ignition, or short trips. The black, powdery soot signals incomplete combustion and poor fuel burn.

Wet Fouling: Usually due to oil entering the combustion chamber, possibly from worn piston rings or valve seals. The black, oily coating prevents proper sparking.

Coolant: Greenish or white chalky deposits suggest coolant is leaking into the combustion chamber, likely from a blown head gasket or cracked head.

Worn: The rounded electrodes and wide gap result from long-term use. This can lead to weak spark and misfires, reducing performance and fuel economy.

P.S. I have a full DIY Mechanic video course (if you are a more video-visual learner), you can join HERE.